The present invention relates generally to electrical slip rings, and more particularly, to an electrical slip ring having a higher circuit density than prior art. Advantageously, the present invention is directed to flat composite electrical slip ring in which the electrical rings are spaced in close proximity to each other.
Electrical slip rings are well known devices for communicating electrical signals from one structural member to another where one of the structural members is rotatable with respect to the other. Such a slip ring apparatus, for example, may comprise a relatively stationary annular base member, which has a plurality of conductive rings extending annularly there around. One or more electrically conductive brushes are arranged on a relatively rotatable structural member to rotate about the stationary annular base member and each of the brushes is arranged to contact a surface of one of the conductive rings thereby forming a series of electrical connections between the two structural members.
A flat or xe2x80x9cpancakexe2x80x9d slip ring is such a device of minimal height or thickness for military or commercial environments where space for the slip ring is very limited. The conductive rings forming the slip ring base generally are formed from materials having a thickness from 0.003 to 0.040 inches with most such materials having a thickness in the range of 0.006 to 0.016 inches. Characteristically, the rings for such a slip ring base are approximately 0.015 to 0.020 inches in width. Spaces between the rings or the ring pitch are characteristically approximately of the same dimension.
A grooved plate process is the most common method of manufacturing pancake slip rings. In the grooved plate process, a grooved plate is prepared by rough machining a brass plate to approximately a xe2x80x9cgrooved platexe2x80x9d shape. The grooved plate is then annealed to minimize distortion during subsequent plastic curing and final machining. One side of the grooved plate is then machined to final xe2x80x9cgrooved platexe2x80x9d shape. Peaks correspond to the bottom of the future rings, and valleys correspond to the future insulation barriers between rings. The xe2x80x9cgrooved platexe2x80x9d is then plated with nickel and a gold strike. A lead wire is soldered or welded to individual ring features on the grooved plate. A glass cloth is then bonded to the plate to prevent leads from entering the valleys of the plate (the future barriers between rings). The plate and lead assembly is then loaded into a metal mold which contains features to provide for internal lead routing, lead exist positioning, and other rotor geometry requirements. The mold is vacuum cast with a liquid epoxy to completely fill the internal detail of the mold. At this point, the assembly is a single piece with a continuous plate on one or two sides with internal epoxy insulation. The final machining step turns the exposed surface of the plate to separate the plate into individual concentric rings separated from each other with epoxy (filling the former valleys in the plate). After the rings are separated, insulating barriers between the rings are machined to final dimensions. In addition, at this step the ring surface is machined to final dimension. The ring groove pattern may be V, U or double-V shaped. In addition to ring shape, the rings are machined to the required surface roughness. The rotor is then nickel plated and then plated with precious metal (usually gold or silver). This process is complex and the density of slip rings is limited by the machining requirements.
An electroformed rings process is another known process. A rotor and lead assembly is prepared by loading lead wire into a mold which contains features to provide for internal lead routing, lead exit positioning, and other rotor geometry requirements. The mold is cast with a liquid epoxy to completely fill the internal detail of the mold and encapsulate the lead wires. Grooves are machined which will contain the rings. Starter rings are prepared as follows. At the bottom of the ring groove, the lead wire conductor is exposed and prepared (generally by applying a fillet of conductive epoxy). The inside walls of the ring groove are coated with conductive plastic to form a continuous conductive starter ring for plating. The ring is electroformed by plating copper onto the starter ring using high build plating technology. High build plating technology or high buildup electroforming is a method of creating a thicker ring cross-section by plating up the starter ring, usually in a copper bath. The starter rings may be plated up with or without dielectric barriers between the rings. At this point, the assembly is a single piece with discrete rings and leads embedded in epoxy insulation. The final machining step will form final shape and texture of the rings and insulating barriers between the rings. The final rotor is nickel plated and then plated with precious metal (usually gold or silver). The disadvantages of the electroformed rings process include limited ring thickness buildup unless barriers are present. Extensive machining is required to create dielectric barriers which allow a buildup of thicker rings. Due to the lengthy times required to electroform the rings, plating solution can damage the slip ring materials, leak into loads embedded in the dielectric causing lead damage and electrical insulation failures. Dielectric materials can interfere with the electroforming process. Ring sides cannot be sealed with nickel allowing corrosion products to form and contaminate the electrical contacts. The contaminants will lead to contact failure and electrical shorts. This is the most significant drawbacks of conventional electroformed rings.
More recent requirements using an electrical slip ring assembly in a Forward Looking Infrared Radar (FLIR) platform have severe space requirements than can be accommodated by either of the grooved plate process or the electroformed rings process. The FLIR systems are used for surveillance, reconnaissance, rangefinder, targeting, and fire control applications. These FLIR platforms all impose severe requirements on the electrical slip ring, including a high circuit density in which many circuits are required and space for the slip ring is always limited. Another requirement is for high bandwidth and low noise for the digitized video signals that pass through the electrical slip ring assembly. Yet another requirement is for low temperature operations in which the electrical slip ring assembly can function over the temperature range of xe2x88x9254xc2x0 C. to +60xc2x0 C.
It is an object of the present invention to provide a flat composite electrical slip ring having a higher circuit density than prior art slip rings.
It is another object of the present invention to provide an electrical slip ring apparatus in which the electrical slip rings are not mechanically machined.
It is yet a further object of the present invention to provide an electrical slip ring which is reliable on the operation, easy to manufacture and cost effective.
These and other objects of the present invention are achieved using three related processes. These processes include double-sided printed circuit board technology, copper electroforming, and chemical machining. Using the present invention, double-sided copper clad glass reinforced epoxy laminate (FR4) is coated with a photosensitive polymer that is imaged using a photographic negative. Following exposure with an intense ultraviolet light source, the photosensitive polymer is then developed with solvent that selectively dissolves away unexposed areas of the resist. When the resist is removed, copper is exposed for subsequent etching.
The photo imaged material is then placed in a copper etchant that removes the exposed copper. Areas protected by the photoresist are unetched and form the interconnecting electrical passages on one side. Holes are subsequently drilled through the etched material to electrically connect the two sides. The connections are formed utilizing plated through-hole processes and/or by filling the holes with a conductive material, such as silver filled epoxy.
Once the through-hole interconnections are formed, photoresist is again applied to both sides of the slip ring circuit boards. A single layer of resist is applied to the electrical interconnects and multiple layers are sequentially applied to the opposite side. The multiple layers of photoresist are then exposed with ultraviolet light through a phototool containing multiple concentric rings. After the image is developed, copper is then electroformed up between the concentric rings of photoresist to form the copper rings. After the resist between the electroformed rings is removed, the electroformed rings are subsequently separated by etching away the thin layer of copper between the base of the rings. Electro formed rings are subsequently recoated with photosensitive polymer and reimaged with a photo tool to allow etching of U-grooves in the ring. These grooves, after a gold allow plating, including small percentage of nickel is applied, form the contact surfaces for the mating brush contacts. The nickel in the allow plating significantly increases the hardness and wear resistance of the gold electrodeposit while maintaining a low electrical contact resistance. The nickel also promotes chemisorption of the lubricant, thus further reducing contact wear.
The slip ring is then mated with brush blocks to form a slip ring apparatus. The resulting slip ring apparatus has a higher circuit density than is readily achievable with conventional slip ring manufacturing methods while having a lower per circuit cost.
The foregoing objects of the present invention are also achieved by manufacturing a slip ring printed circuit board including forming a plurality of concentric spaced electrical contacts on one side of a non-conductive base and forming interconnecting electrical paths on an opposite side of the non-conductive base. Manufacturing a slip ring printed circuit board also includes electrically connecting the electrical contacts and the interconnecting electrical paths, depositing copper on the electric contacts to form electrical rings and etching a groove into each of the electrical rings.
The foregoing objects of the present invention are also achieved by a method of manufacturing a slip ring printed circuit board includes forming a plurality of concentric spaced electrical contacts on one side of a non-conductive base and forming interconnecting electrical paths on an opposite side of the non-conductive base. A method of manufacturing a slip ring printed circuit board also includes electrically connecting the electrical contacts and the interconnecting electrical paths, depositing copper on the electrical contacts to form electrical rings and etching a groove into each of the electrical rings.
The foregoing objects of the present invention are also achieved by an electrical slip ring apparatus includes an annular base member and at least one brush block assembly secured to the annular base member having a plurality of brushes. A flat composite electrical slip ring includes an electrically non-conductive base and a plurality of concentric spaced electrical rings located on one side of each of the electrically non-conductive base. The slip rings are spaced from adjacent electrical rings at a distance of approximately 0.70 or greater. Interconnecting electrical paths are located in an opposite side of the electrically non-conductive base. Connecting means are provided for connecting at least some of the electrical rings to the interconnecting electrical paths.
The foregoing objects of the present invention are also achieved by a flat composite electrical slip ring product produced by the method includes forming a plurality of concentric spaced electrical contacts on one side of a non-conductive base and forming interconnecting electrical paths on an opposite side of the non-conductive base. The foregoing objects of the present invention are also achieved by a method of manufacturing a slip ring printed circuit board also includes electrically connecting the electrical contacts and the interconnecting electrical paths, depositing copper on the electrical contacts to form electrical rings and etching a groove into each of the electrical rings.
Still other objects and advantages of the present invention will become readily apparent to those skilled in the art from the following the detailed description, wherein the preferred embodiments of the invention are shown and described, simply by way of illustration of the best mode contemplated of carrying out the invention. As will be realized, the invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, all without departing from the invention. Accordingly, the drawings are to be regarded as illustrative in nature, and not as restrictive.